The subject matter of the present invention is a method of draining condensate from a reservoir of a compressed air or compressed gas system that can be subjected to a pressure above atmospheric, to a subatmospheric pressure or to atmospheric pressure depending on the operating condition of the compressed air or compressed gas system and a device for carrying out this method.
The method of the invention and the device of the invention are more particularly provided for the purpose of draining condensate from a reservoir of a multistage air or gas compressor, which will be termed gas compressor hereinafter. The method of the invention and the device of the invention will be explained hereinafter in the light of their application on multistage gas compressors.
Depending on the operating conditions compressed gas compressors always take in a certain quantity of water vapor that partially condenses after the compression process. This happens to a greater extent when an aftercooler is arranged downstream of a gas compressor, the higher temperatures in the compressed gas resulting from the compression process, which typically may amount to more than 100xc2x0 C., being reduced to technically sensible values in said aftercooler.
In a multistage gas compressor, compression is carried out step-by-step in compressor stages connected in series. Each compressor stage is hereby typically provided with a compressor unit of its own, an aftercooler and a trap arranged downstream thereof. The condensate produced in aftercooling is separated from the compressed gas stream and is led out of the gas compressor. The pressure of the compressed gas or gravitation generally cause thereby the condensate to be discharged via a drain pipe into a reservoir of any configuration. Said reservoir is thereby either a component part of the gas system or a component part of a condensate drain device by means of which the collected condensate is eventually discharged from the gas compressor.
On account of the particular operating conditions that may occur in multistage gas compressors, the use of previously known condensate drain devices with a pilot operated diaphragm valve for draining collected condensate from a reservoir as they have been described in EP 391 250 for example is possible to a limited extent only. The reason therefore is that, on account of the operating conditions of a gas compressor, both excess pressure and negative pressure as well as atmospheric pressure can prevail.
The use of condensate drain devices with float actuated exhaust valves is not advantageous because they are prone to dirt and grime, dirt and grime being inevitable in compressed gas plants.
For reasons of economy, it does not make any sense to operate a multistage gas compressor at full output power against atmospheric pressure when no compressed gas is demanded by a consumer. Generally, it does not make sense either to switch off the multistage compressor since each process of switching off and of starting implies increased wear of the motor and the compressor. Therefore, in case the demand for compressed gas is reduced, the compressor is often set to idle in such a manner that a sealing device locks the intake side of the first compressor except for a small opening cross-section while an exhaust device located on the pressure side of the last compressor stage is concurrently opened. The exhaust device can be connected to the intake side of the first compressor. A locking device provided on the pressure side of the last compressor prevents the already produced compressed gas from flowing back into the compressor. This arrangement permits the multistage compressor to continue to run without load at low pressure at idle, which considerably saves energy. Increased wear, as it is caused by frequent switching off and on can thus be avoided.
If the compressor is set to the idle condition described, the compressor continues to run without load at low pressure. But a small quantity of compressed gas is delivered, a certain pressure ratio between the various compressor stages being maintained as a result thereof. If the multistage compressor is in this state of control for a longer period of time, pressures below the pressure in the intake gas, more specifically below atmospheric pressure, may develop in the first compressor stages.
If no compressed gas is needed for an extended period of time, the whole multistage compressor can be switched off in order to save the energy still needed for operating at idle. Once all of the compressor units have been stopped, the compressor unit is generally vented with atmospheric pressure via an exhaust device on the pressure side of the last compressor stage. In this case, atmospheric pressure prevails in all the parts of the multistage compressor.
As a result thereof, different operating conditions can occur in a reservoir in which condensate produced in a trap of a compressor unit is collected. At normal operation of the multistage compressor, all of the reservoirs that are arranged on the pressure side of a respective one of the compressors are subjected to pressures above the pressure of the intake gas, more specifically above atmospheric. In this case, collected condensate could be discharged from all the reservoirs by means of condensate drain devices built according to previously known design principles. Drainage thereby is performed either driven by the excess pressure relative to atmospheric pressure prevailing in the compressor stage and/or by gravitation.
The same applies when the multistage compressor is completely turned off. In this case, all of the reservoirs are subjected to atmospheric pressure so that the condensate can be drained by gravitation from all the reservoirs without any problem. In this case too, prior art condensate drain devices can readily be utilized inasmuch as a certain excess pressure is not needed for opening.
As already described herein above though, in the described idle condition when less compressed gas is demanded, some compressor stages located in proximity to the gas intake (i.e., low compressor stages) may be subjected to pressures below the intake pressure, more specifically below atmospheric. In this case, the condensate produced in a trap of such compressor stages is drained into a reservoir, the collected condensate with the gas space located there above being however subjected to a pressure below the initial pressure, more specifically below atmospheric.
In using a condensate drain device as disclosed in EP 391 250 for draining the condensate the problem is that, due to their design, these condensate drain devices open their exhaust valve when a negative pressure develops in the reservoir and the pressure difference between reservoir and exhaust side of the condensate drain device, which is subjected i.a. to atmospheric pressure, reaches a certain value. The negative pressure in the reservoir and in the compressor stage causes ambient air to be drawn into the amplifier stage and condensate may be simultaneously entrained. Various compressor units may thus be vented on their exhaust side, which is not desired, and concurrently slurried with condensate, which may damage the compressor units. This is the reason why the condensate must in any case be prevented from being discharged from a reservoir which is subjected to a pressure below atmospheric.
The same applies when trying to drain the collected condensate from the reservoir subjected to negative pressure under the influence of gravity by means of a conventional condensate drain device.
In principle, it is possible to discharge collected condensate from the reservoirs of a multistage compressor by means of conventional condensate drain devices, by those described in EP 391 250 for example. Additional safety measures must thereby be taken in order to prevent drainage against atmospheric pressure when a reservoir is subjected to negative pressure. For this purpose, a check valve is often arranged behind the condensate drain device, said check valve reliably preventing the condensate drain device from undesirably opening and ambient air from penetrating into the reservoir subjected to negative pressure.
When the multistage compressor is turned off, the reservoir is subjected to atmospheric pressure. In using previously known condensate drain devices with a pilot operated diaphragm valve according to EP 391 250 for example for draining the condensate, the problem is that the pilot operated diaphragm valves used for drainage require a certain excess pressure in the reservoir relative to the pressure on the exhaust side, that is to say i.a. atmospheric pressure. As a result thereof, when the gas compressor is turned off, it is no longer possible to drain condensate. This however represents a risk since certain quantities of condensate may still be produced even when the plant is at standstill, at least immediately after having turned it off.
It is therefore the object of the present invention to indicate a method that permits to drain condensate from a reservoir that is subjected, in function of the operating condition of the compressed gas system assigned thereto, to an excess pressure, a negative pressure or atmospheric pressure. Operating conditions in which ambient air may penetrate, possibly mixed with condensate, through the reservoir into the compressed gas plant, must reliably be prevented.
It is another object of the invention to indicate a method that permits condensate to be discharged from a multistage compressor at load operation and when said compressor is switched to an off condition, said method being suited to be developed in an simple manner so that drainage is also possible in such operating conditions of the compressor in which the reservoir is subjected to a negative pressure.
It is eventually also an object of the present invention to indicate a condensate drain device for carrying out said method.
It is more specifically an object of the invention to indicate a condensate drain device for a multistage compressor that permits at load operation and when said compressor is switched to an off condition to reliably drain the condensate generated and to reliably prevent, in the idle mode, ambient air from being taken in, possibly mixed with condensate, on the pressure side of a compressor unit. Preferably, this is to be realized with the least possible expenditure in switching technique. Furthermore, a condensate drain device in accordance with the invention is to be developed in such a manner that the condensate can be reliably drained even in case the multistage compressor operates at idle.
The method of the invention can be used with any compressed gas system in which condensate is generated that is collected in a reservoir for condensate, said reservoir being subjected to an excess pressure, a negative pressure or atmospheric pressure depending on the operating condition of the compressed gas system.
The method involves the following steps:
a) collecting condensate in the reservoir,
b) acquiring the fill level of condensate in the reservoir by means of a level meter and an electronics unit arranged downstream thereof,
c) providing a control pressure p(control) which, in any operating conditions of the compressed gas system, is above the pressure p(reservoir) in the reservoir and above atmospheric pressure p(air),
d) providing, under the control of the electronics unit (26), a control conduit input (241) of an exhaust valve (24) which is assigned to the reservoir (601) and is intended to drain the condensate collected in the reservoir (601) with a pressure p(input)
the pressure p(input)
i) substantially corresponding to the control pressure p(control) when the maximum fill level in the reservoir has not yet been reached or exceeded,
ii) substantially corresponding to the atmospheric pressure p(air) when the maximum fill level in the reservoir is reached or exceeded,
and wherein the position of the exhaust valve is controlled by the pressure p(reservoir) in the reservoir, the pressure p(input) at the control conduit input, the pressure on the exhaust side of the exhaust valve, which more specifically can be atmospheric pressure p(air), and by a threshold value P, more specifically wherein the exhaust valve for draining the condensate
iii) is closed when the pressure p(input) is above the pressure p(reservoir) in the reservoir plus the threshold value P and
iv) is opened when the pressure p(input) is below the pressure p(reservoir) in the reservoir plus the threshold value P.
More specifically, the method may be used with any multistage compressor being provided with several compressor stages connected in series, more specifically with at least two such stages. At least the compressor unit that operates at lower pressures must be provided with a trap.
The method is particularly suited for use with a multistage compressor for compressing a gas, e.g., air, that is provided with compressor stages connected in series, each of the stages being comprised of one compressor unit n with an aftercooler and a trap, both of which are arranged downstream of said compressor unit. The condensate generated in the trap is drained via a drain pipe for example into a reservoir of any design. The gas pressure p(reservoir) prevailing in the gas space located above the condensate in the reservoir substantially corresponds to the gas pressure p(i) prevailing on the exhaust side of the corresponding ith compressor stage. In this case, the method advantageously uses as a control pressure p(control) substantially the gas pressure p(j) on the exhaust side of the compressor stage j. When the multistage compressor is in operation, the jth compressor stage reaches a higher gas pressure p(j) than the ith compressor stage which reaches, on the exhaust side, the gas pressure p(i).
In selecting the appropriate threshold value P it can be made certain that, in all the operating conditions of the compressed gas system, condensate is only discharged from the reservoir when the fill level of the condensate in the reservoir has reached or exceeded a maximum value and when the reservoir is not subjected to a negative pressure relative to the exhaust side of the exhaust valve i.e., i.a. atmospheric pressure.
The threshold value P is preferably selected in such a manner that in any operating conditions (at load, at idle) of the compressed gas system the following applies:
a) p(reservoir)+P less than p(input) when p(input)=p(control), [p(control) being greater than p(air)],
b)
i) p(reservoir)+P greater than p(input), when p(input)=p(air) and p(reservoir)xe2x89xa7p(air), more specifically p(reservoir)=p(excess)
ii) p(reservoir)+P  less than p(input) when p(input)=p(air) and p(reservoir)=p(negative), preferably p(reservoir) less than p(air)
The characters indicating a relationship mean xe2x80x9csubstantially correspond toxe2x80x9d. Furthermore, when the threshold value P is predetermined by e.g., a spring (as it will be described hereinafter), the control pressure p(control) can also be chosen accordingly.
A device needed for carrying out said method requires but a minimum of electric appliances, as the whole method can be carried out with a condensate drain device operating almost entirely mechanically with compressed gas. Outstanding advantages with regard to reliability and freedom from maintenance of the structure and a large indifference to possible power supply failures derive therefrom.
For carrying out the method of the invention, a condensate drain device provided with the following component parts is suited:
a) a level meter for acquiring the fill level of condensate in the reservoir, more specifically for acquiring the condition in which the maximum fill level in the reservoir has been reached or exceeded,
b) a control valve arranged in a control conduit via which a control pressure p(control) is delivered to the condensate drain device, said control pressure being above the pressure in the reservoir p(reservoir) under any operating conditions of the compressed gas plant, i.e.,
p(control) greater than p(reservoir),
in any case however above atmospheric pressure p(air), the control valve being provided for controlling the flow of the control pressure p(control) through the control conduit,
c) an exhaust valve assigned to the reservoir and provided for discharging condensate from the reservoir, more specifically against atmospheric pressure p(air), the exhaust valve being configured in such a manner that it is provided with a control conduit input which is connected to the control conduit by way of the control valve so that, when the control valve is open, the control pressure p(control) prevails at the input of the control conduit,
d) a venting valve which is arranged in the control conduit between control valve and control conduit input and which, when the control valve is closed, vents the control conduit between control valve and control conduit input until it reaches atmospheric pressure p(air),
e) an electronics unit that interprets the signal of the level meter and closes the control valve when the maximum fill level in the reservoir has been reached or exceeded,
the exhaust valve being provided with the following positions of control:
i) a locking position for condensate when the pressure prevailing at the input of the control conduit p(input) is higher than the pressure prevailing in the reservoir p(reservoir) plus a preset threshold value P,
ii) a flow through position for condensate when the pressure prevailing at the input of the control conduit p(input) is lower than the pressure prevailing in the reservoir p(reservoir) plus threshold value P.
In accordance with the method described herein above, the device can be realized in such a manner that, in selecting the proper threshold value P, the condensate drain device of the invention only discharges condensate from the reservoir when the pressure in said reservoir is at least atmospheric p(air) or above p(excess).
Generally, the threshold value P must be adjusted to the pressures developed under diverse operating conditions of the multistage compressor. More specifically, the threshold value P can also be selected in such a manner that the exhaust valve is opened only when at least a slight excess pressure prevails in the reservoir. Actually however, this is not provided for, but may be advantageous or even necessary in certain individual cases.
If the pressure p(negative) in the reservoir is below atmospheric p(air), the drive of the exhaust valve reliably prevents the reservoir from being vented with atmospheric pressure by the condensate drain device of the invention, the corresponding compressor unit being thus reliably prevented from undesirably being vented and more specifically from being slurried with condensate.
More specifically, the condensate drain device in accordance with the invention is configured in such a manner that, irrespective of the fill level of the condensate, the condensate drain device reliably keeps the exhaust valve closed when the pressure in the reservoir is negative.
Even without additional check valve, the valve cannot open automatically because of the difference in pressure between the exhaust side of the exhaust valve and the reservoir. This constitutes a substantial improvement over the previously known condensate drain devices with pilot operated diaphragm valves as they have been disclosed in EP 391 250 for example.
Due to its structure, the exhaust valve cannot be opened either when the reservoir is subjected to a negative pressure. Faulty operation is thus excluded and the compressed gas system is reliably prevented from being slurried with condensate even without a check valve being arranged downstream thereof.
Finally, condensate can also be discharged from the reservoir when said reservoir is subjected to atmospheric pressure. The exhaust valve is designed in such a manner that it automatically opens the passage for condensate when atmospheric pressure prevails in the reservoir and at the input of the control conduit, which is the case when the compressed gas system is at standstill.
For a better understanding it should be noted that, when the pressure p(reservoir) is used as the control pressure p(control) and when the threshold value P is selected accordingly, the condensate drain device of the invention substantially operates like a conventional condensate drain device as disclosed in EP 391 250 for example. The threshold value P is selected such that the exhaust valve only opens when a certain excess pressure p(excess) prevails in the reservoir i.e., the exhaust valve is closed when there is equilibrium pressure between the reservoir, the input of the control conduit and the exhaust side of the exhaust valve. By contrast, with the condensate drain device of the invention, the exhaust valve is open when there is equilibrium pressure between the reservoir, the input of the control conduit and the exhaust side of the exhaust valve.
In developing the condensate drain devices disclosed in EP 391 250 in accordance with the method of the invention in such a manner that a condensate drain device is realized in accordance with the invention, it is made possible to drain condensate by means of a type of a condensate drain device that proved its worth in practical use even in the difficult case of use in which condensate is to be drained from a reservoir that is subjected, depending on the operating condition of the corresponding compressed gas system, to a negative pressure, an excess pressure or to atmospheric pressure. In this advantageous development of the device of the invention, the exhaust valve is configured as a compressed air actuated diaphragm valve. The diaphragm valve is thereby driven by the pressure prevailing at the input of the control conduit p(input), by the pressure in the reservoir p(reservoir), by the pressure on the exhaust side of the exhaust valve, i.e., i.a. atmospheric pressure p(air), and by an additional force K which represents the afore mentioned threshold value P.
This force K can more specifically be realized by means of a spring provided in the exhaust valve. In order to permit adjustment of the threshold value P to the conditions of the compressed gas compressor, the threshold value P is preferably settable at the very exhaust valve. This can be realized in that e.g. either the force of the spring or its bias is variably adjustable.
It is no longer necessary to possibly turn, as an alternative, to condensate drain devices with float actuated exhaust valves which are prone to dirt and wear.
Particular advantages are obtained when the electronics unit, which interprets the signal of the level meter, closes the control valve as soon as the maximum fill level in the reservoir is exceeded and additionally maintains, after closure (i.e., after the maximum fill level has been exceeded), the control valve closed or the exhaust valve open until the reservoir is emptied as completely as possible. Such a control has become known to be provided in the condensate drain devices described in EP 391 250 or DE 196 45 815 so that it needs not be explicitly described here. More specifically, such a control may make use of a second level meter for acquiring a lower fill level and/or a timing circuit. The device of the invention can be connected to all the prior art controls that serve this purpose.